This application claims Paris Convention priority of DE 100 60 284.3 filed Dec. 5, 2000 the complete disclosure of which is hereby incorporated by reference
The invention concerns a magnet arrangement comprising an actively shielded superconducting magnet coil system for generating a magnetic field in the direction of a z axis in a working volume disposed on the z axis about z=0, and with a plurality of protective elements for protecting the superconducting magnet coil system in the case of a breakdown in the superconducting state thereof (=quench), wherein the superconducting magnet coil system comprises a radially inner partial coil system and a radially outer partial coil system which are electrically connected in series, arranged coaxially with respect to one another, and which each produce a magnetic field in the working volume, of opposite directions along the z axis, wherein the superconducting current path of the magnet coil system is divided into several sections which are each electrically connected in parallel with at least one of the protective elements, wherein at least one of these sections comprises windings of the radially inner partial coil system and windings of the radially outer partial coil system.
U.S. Pat. No. 5,644,233 discloses a magnet arrangement comprising an actively shielded magnet coil system with a radially inner and a radially outer partial coil system, wherein the magnet coil system is divided into several sections each comprising windings of the inner partial coil system and windings of the outer partial coil system, such that the dipole moment of each of these sections is approximately zero, wherein each of the sections is connected in parallel with a protective element.
Superconducting magnets are used for different applications including, in particular, high field applications, e.g. for magnetic resonance methods. High field magnets of this type generally produce a considerable stray field which can be dangerous for the surroundings of the magnet. This problem can be solved by providing the magnet with an active shielding, i.e. with an additional superconducting coil which is connected in series with the main coil of the magnet, however, which produces a field of opposite polarity.
In addition to strong stray fields, another problem with superconducting high field magnets is the risk of a sudden breakdown of the superconducting state (=quench). To protect the superconducting wire of the magnet from overheating and from being destroyed in case of a quench, superconducting magnets usually comprise a device which diverts the magnetic current from the coil sections which have become resistive during a quench and through protective elements, e.g. resistances. For actively shielded magnets, such a quench protection device can cause the following problem: Should e.g. the shielding coil quench, the current flows away from the shielding via the protective elements connected in parallel while substantially the same original magnet current continues to flow in the main coil. This causes an extremely large short-term increase in the stray field of the magnet arrangement compared to the stray field during normal operation.
In an arrangement in accordance with U.S. Pat. No. 5,644,233, the problem of a possibly excessive stray field during a quench is solved in that each coil section, connected in parallel with its own common protective element, is composed of parts of the main coil and the shielding coil such that the dipole moment of each section during operation is small. If a quench occurs in such a section thereby locally reducing the current, the dipole moment of the magnet arrangement will, at most, change slightly. For this reason, the stray field cannot increase considerably.
However, it is not always possible to provide all coil sections bridged by protective elements with negligible dipole moments, in particular, for high field magnets. There are two main reasons therefore. Firstly, for each section, a superconducting connection must be provided between the parts of the main coil and the shielding portions. This is difficult to realize when the magnet is finely divided into many sections. Secondly, the dipole moments of the main coil and shielding portions cannot be exactly matched to each other since the beginning and end of the sections cannot lie within a winding packet but only at the end of an entire coil layer. Therefore, typical high field magnets unavoidably have at least some of the sections bridged with protective elements which have a dipole moment which is considerably different from zero. This can cause a considerable change in the stray field of the magnet arrangement during a quench in one of these sections.
In contrast thereto, it is the object of the present invention to considerably reduce the danger of an excess stray field during a quench in a magnet arrangement having at least one section which is bridged with a protective element and which has a non-negligible dipole moment.
This object is achieved in accordance with the invention in that at least one additional closed current path is provided which has a non-vanishing areal winding number and a non-zero inductive coupling LAi⇄C3 to at least one section Ai, in particular, such that the coupling coefficient       K    i    =            L              Ai        ↔        C3                                      L          Ai                ⁢                  L          C3                    
is larger than 0.01, wherein LAi and LC3 characterize the self-inductances of the section Ai and of the additional current path C3, respectively.
The dipole moments of the sections bridged by a protective element are reduced as much as possible through combination of coil portions from the main coil and the shielding coil. Moreover, an additional current path with a non-vanishing areal winding number is also provided which is separate from the magnet coil system and which is inductively coupled to at least one of those sections whose dipole moment clearly differs from zero. In case of a quench in one of these sections, a current is induced in the additional current path which compensates for the change in the dipole moment of the magnet arrangement occurring during a quench, thereby keeping the stray field largely constant, or even reducing it, during the quench.
The advantage of an inventive arrangement consists in that, when forming the sections bridged with a protective element, the dipole moments of the sections must not necessarily be completely negligible, good protection from excess stray fields during a quench of the magnet arrangement is nevertheless still ensured. This permits a more flexible design of the quench protection device to facilitate production thereof or it can be dimensioned to provide optimum protection for the coil with regard to overheating during a quench.
One embodiment of the inventive magnet arrangement is particularly advantageous wherein the radially inner and outer partial coil systems have approximately equal magnetic dipole moments of opposite sign. Such a magnet arrangement has optimum magnetic shielding (i.e. a very small stray field) due to the substantially negligible magnetic dipole moment of the overall system. In such an arrangement, it would be particularly difficult to maintain the low stray field values during a quench, solely through suitable selection of the sections bridged by a protective element.
In one advantageous embodiment of the inventive magnet arrangement, the magnet arrangement is part of an apparatus for high-resolution magnetic resonance spectroscopy. In such magnet arrangements, the radially inner partial coil system generally has a very large dipole moment due to the required high field strengths, and therefore, the use of actively shielded magnet systems is particularly beneficial. The inventive arrangement guarantees that there are no excess stray fields in case of a magnet coil system quench, which would be particularly unfavorable in this case due to the generally large dipole moment of the main coil.
In a particularly preferred embodiment of the inventive magnet arrangement, the additional current path is formed by an additional coil system which is not electrically connected with the magnet coil system and which is arranged coaxially thereto. Selection of an additional coil system permits maximum flexibility when dimensioning the additional current path, thereby achieving optimum protection against excess stray field during a quench of the magnet coil system.
In a further preferred embodiment of the inventive magnet arrangement, the additional current path consists of a part of the magnet coil system bridged with an additional switch. This provides protection against excess stray fields during a quench without requiring large manufacturing effort.
In one advantageous embodiment of the inventive magnet arrangement, the additional current path has a small ohmic resistance R3, preferably such that the time constant of the additional current path LC3/R3 is longer than one secondxe2x80x94wherein LC3 is the self-inductance of the additional current path. This guarantees slow reduction of current induced in the additional current path with, however, a time constant which is much larger than the typical time constant of a quench of the magnet coil system, thereby ensuring compensation of the magnetic dipole moment change during a quench.
In a further advantageous embodiment of the inventive magnet arrangement, the additional current path is superconducting. This variant has the advantage that the currents induced during a quench are not reduced before the entire magnet arrangement has quenched.
In one advantageous embodiment of the inventive magnet arrangement, the additional current path comprises a device for limiting the current induced therein. This prevents possible induction of excessive current in the additional current path, which could damage same.
Three further advantageous embodiments of the inventive magnet arrangement are characterized in that the additional current path is part of a device which provides the magnet arrangement with an additional function. This additional function can be, in particular, a drift compensation of the magnetic field of the magnet arrangement in the working volume, a shim device or compensation of external magnetic field fluctuations. This double function of these embodiments advantageously permits more compact design of the overall magnet arrangement.
A particularly preferred embodiment of the inventive magnet arrangement is characterized in that the additional current path is inductively decoupled from the magnet coil system. This is advantageous in that, when charging or decharging the magnet coil system, no current is induced in the additional current path and after a quench of the magnet coil system, no residual current flows in the additional current path even if the additional current path is superconducting.
In a further particularly preferred embodiment of the inventive magnet arrangement, at least one of the sections having an inductive coupling with the additional current path which is substantially different from zero, also has a magnetic dipole moment substantially different from zero, wherein the current which is induced in the additional current path in case of a quench in this section, produces a magnetic dipole moment which corresponds substantially to that of this section before the quench. This guarantees that the magnetic dipole moment of the magnet arrangementxe2x80x94and therefore its stray fieldxe2x80x94is maintained during a quench in this section.
In a particularly preferred further development of the above embodiment, the additional current path is superconducting and thermally decoupled from at least one of the sections. The amount of heat transferred to the additional current path during a quench in this section is therefore small.
In a particularly advantageous embodiment, an amount of heat transferred to the additional current path is not more than 50% of that amount of heat required in the additional current path for triggering a quench. This embodiment is of particular interest if the section thermally decoupled from the additional current path has a substantial magnetic dipole moment which is transferred to the additional current path through inductive coupling during a quench. In this case, one guarantees that the magnetic dipole moment produced by the additional current path is not removed by a quench of the additional current path produced by thermal coupling with the quenching section, since this would lead to an increase in the stray field of the magnet arrangement.
Another preferred further development of the inventive magnet arrangement is characterized in that the additional current path is superconducting and thermally coupled to at least one of the sections, preferably such that the amount of heat transferred to the additional current path during a quench corresponds to at least twice the amount of heat required for triggering a quench in the additional current path. This embodiment is of particular interest if the section which is thermally coupled to the additional current path has a negligible magnetic dipole moment and a substantially negligible inductive coupling with the additional current path. This guarantees that, during a complete quench of the magnet coil system, the additional current path also quenches thereby suppressing generation of a stray field.
In a further embodiment of the magnet arrangement, at least one of the sections consists mainly of windings of the radially inner partial coil system or mainly of windings of the radially outer partial coil system, and preferably such that this section consists only of windings of either the radially inner partial coil system or of the radially outer partial coil system. In this embodiment, the particularly sensitive sections of the magnet coil systemxe2x80x94e.g. the radially innermost windings of the high field magnetsxe2x80x94are advantageously optimally protected. In these cases, the formation of several smaller sections which exclusively comprise windings of the radially inner partial coil system is desirable.
In another particularly preferred further development of the inventive magnet arrangement, the magnet arrangement additionally comprises a passive magnetic material shielding. This permits suppression of stray field fluctuations generated during a quench, thereby enhancing the stray field-reducing effect of the inventive arrangement.
In a particularly preferred embodiment of the inventive magnet arrangement, the passive shielding is an integral component of a cryostat in which the magnet coil system is located. The constructive integration of the passive shielding in the cryostat permits more compact design of the overall magnet arrangement.
In another preferred further development of the inventive magnet arrangement, the passive shielding is not completely magnetically saturated during operation of the magnet coil system. This guarantees that the remaining additional possible magnetization of the passive shielding can be used to compensate for an excessive stray field generated during a quench.
In a particularly preferred embodiment of the inventive magnet arrangement, the passive shielding consists of ferromagnetic material, in particular of soft iron. The high permeability, high saturation magnetization, good commercial availability and low price of soft iron permits production of a very inexpensive and effective additional protection from excess stray fields during a quench of the magnet arrangement.
In one last advantageous embodiment, the passive shielding is disposed radially beyond the radially outermost winding of the magnet coil system (M).
Further advantages of the invention can be extracted from the description and the drawing. The features mentioned above and below can be used in accordance with the invention either individually or collectively in any arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration, rather have exemplary character for describing the invention.
The invention is shown in the drawing and further explained by means of embodiments.